X-ray tubes are widely used in medical diagnosis and treatment, industrial manufacturing, testing and inspection, security control, and a variety of other applications. An x-ray tube typically includes a cathode assembly having an electron source and an anode assembly having a target, both disposed within an evacuated enclosure. The target is oriented to receive electrons from the electron source. In operation, an electric current is applied to the electron source such as a filament, causing electrons to emit by thermionic emission. The electrons are then accelerated towards the target surface by applying a high voltage potential between the cathode and the anode. Upon striking the anode target surface, some of the resulting kinetic energy is released as x-rays. The x-rays ultimately exit the x-ray tube through a window in the x-ray tube, and interact in the patient or other objects for applications such as medical diagnostic and treatment, sample analysis, or various other applications.
X-ray tubes are typically operated under high temperatures, high voltages, and high vacuum conditions. For example, the operating temperature of an x-ray tube can be as high as 1300° C. The thermal stresses imposed by high operating temperature and temperature gradient often have various detrimental effects on the structure and performance of the cathode, the anode, and various other components in the x-ray tube. One area where such thermal effects are of particular concern relates to high voltage cables, which are employed in the x-ray tube to provide a high voltage potential between the cathode and the anode, and to power the filament for operation of an x-ray tube. Typical high voltage cable includes a cable having one or more electrical conductors electrically isolated from each other and wrapped in a protective covering or sheath. At an end of the cable is a terminal which typically includes a rubber element or rubber plug. The high operating temperature may impose detrimental effects on the rubber element, causing e.g. degradation of the electrical cable and potential high voltage failures.
An issue related with the thermal stresses is the high vacuum operating environment in the x-ray tube. Generally, the enclosure within which the cathode and the anode are disposed is evacuated to a relative high vacuum in order to ensure the removal of gases and other materials that may cause arcing due to the high potential difference between the cathode and the anode. However, thermal energy cannot be transferred by convection in vacuum since there contains no fluids or matters that are needed for transferring heat by convection. In some applications, the electron source e.g. filament in a cathode assembly is on continuously, creating a steady heat source. Conventional cathode assemblies employ polished cathode shields, which have low emissivity values. Therefore, conduction of heat from the filament heat source to the receptacle of the power cable is the primary path of heat transfer, causing concerns of high voltage failures.
Accordingly, there is a need for cooling x-ray tubes in general to ensure reliable operation under extreme conditions for sustained periods of time. There is a need for cooling cathode assemblies by emissive heat transfer to x-ray tube envelopes in order to eliminate or mitigate the detrimental effects on the x-ray tube components and to enhance the overall performance of the x-ray tube.